1. Field of the Invention
The present invention relates to vibratory angular sensors and more specifically to an anomaly detector for the vibratory angular sensors.
2. Description of the Related Art
A known vibratory angular rate sensor is provided with a differential amplifier for producing a differential output indicating the difference between the outputs of a pair of sensing elements (or gyro-sensor). A predetermined frequency component is detected from the differential output using a synchronous detector and a low-pass filter. The differential output has a DC offset voltage, which win become abnormal when the bonding wire between the sensor and the differential amplifier is broken, for example. For this reason, an anomaly detector 2, as shown in FIG. 1A, has been developed for a vibratory angular rate sensor 1 for detecting when the offset voltage of the differential output goes abnormal. The known anomaly detector 2 is comprised of a low-pass filter 3 for filtering the differential output of the sensor 1. The output of the low-pass filter 3 is compared in a window comparator 4 with an upper threshold voltage VRH and a lower threshold voltage VRL. When the angular rate sensor is operating properly, the low-pass filter output lies between the upper and lower threshold voltages and a high-level output is delivered from the window comparator 4. When the sensor is not operating properly, the low-pass filter output rises above the upper threshold voltage or falls below the lower threshold voltage, and the window comparator 4 produces a low-level, warning signal.
More specifically, the differential output of the angular rate sensor 1 is represented as:VDIF(t)=Va sin ωdt+Vdc  (1)
where ωd=2πfd and Vdc is the DC offset voltage. The low-pass filter output VLPF is equal to the offset voltage Vdc if the low-pass filter is of a non-inverting type as shown FIG. 1A and equal to 2×VREF−Vdc if the low-pass filter is of an inverting type as shown in FIG. 1B, where VREF is the reference voltage which is impressed on the inverting input of the operational amplifier and corresponds to the midpoint between the upper and lower threshold voltages VRH and VRL of the window comparator 4. As shown in FIG. 2, the window comparator 4 compares the output voltage VDIF(t) of either low-pass filter 3 with the reference voltages VRH and VRL and determines whether the difference VDIF(t)−VREF=ΔVdc is outside the range between VRH and VRL. If this is the case, the sensor is abnormal and the window comparator 4 produces a warning signal.
The output of the low-pass filter 3 contains a ripple component (with frequency equal to fd), which is desired to be as small as possible. Further, it is necessary that the cutoff frequency (fc) of the low-pass filter 3 is sufficiently lower than the frequency fd of the input differential voltage. For these reasons, low-pass filter 3 must be designed with a large time constant value. However, the use of large time-constant low-pass filter will introduce a timing delay in detecting an abnormal offset voltage. If a gradually varying anomaly occurs, the prior art detector cannot quickly detect the fault due to the introduced timing delay. Further, with such a low-pass filter the time constant value may be large in comparison with the varying rate of a transient abnormal offset voltage. If such a transient anomaly occurs in a period smaller than the time constant of low-pass filter 3, the prior art anomaly detector 2 would fail to detect the anomaly.